Wie kann ich verschachtelte referenzierte Modelle füllen?

Zum Beispiel:

// 'Collection' model
var CollectionSchema = new Schema({
  collection_name: String,
  _groups: [{ type: Schema.Types.ObjectId, ref: 'Group' }],
});

// 'Group' model
var GroupSchema = new Schema({
  group_name: String,
  _item: { type: Schema.Types.ObjectId, ref: 'Item' }  // To be populated
  _meta: [ { type: Schema.Types.ObjectId, ref: 'Meta' } // To be populated
});

// 'Item' model
var ItemSchema = new Schema({
  item_name: String,
  item_description: String
});

// 'Meta' model
var MetaSchema = new Schema({
  meta_name: String,
  meta_value: String
});

Ich möchte jedes "_item" in "_group" in "Collection" -Modellen einfügen. Ich meine so etwas zu bekommen:

{
 collection_name: "I'm a collection"
 _groups: [ 
           { _id: ObjectId("520eabd1da5ff8283c000009"),
              group_name: 'Group1',
              _item: { _id: ObjectId("520eabd1da5ff8283c000004"),
                        item_name: "Item1 name",
                        item_description: "Item1 description"
                    },
             _meta: {
                       _id: ObjectId("520eabd1da5ff8283c000001"),
                      meta_name= "Metadata name",
                      meta_value = "metadata value"
                    }
            },
            { _id: ObjectId("520eabd1da5ff8283c000003"),
              group_name: 'Group2',
              _item: { _id: ObjectId("520eabd1da5ff8283c000002"),
                        item_name: "Item2 name",
                        item_description: "Item2 description"
                    },
             _meta: {
                       _id: ObjectId("520eabd1da5ff8283c000001"),
                      meta_name= "Metadata name",
                      meta_value = "metadata value"
                    }
            }
          ]
}